The present invention relates to a system and method for removing NOx from an emission control device coupled to an engine.
Description of the Related Art
It is known to utilize a NOx trap to reduce NOx emissions from an engine. One such engine and NOx trap is disclosed in Japanese Patent 06-272541A. In particular, an engine control system injects a reductant in the exhaust gases upstream of the NOx trap to reduce NOx stored in the trap.
The known engine control strategy, however has a significant drawback. The injected liquid reductant is composed of a plurality of droplets and therefore the reductant is not well mixed with the exhaust gases, as compared to having a vaporized reductant which would be well mixed with the exhaust gases. As a result, the reductant is unevenly distributed within an emission control device such as a NOx trap. For example, a reductant droplet contacting a particular area within the NOx trap will reduce stored NOx at that area. However, relatively large areas of the NOx trap, not contacted by the droplets, will not have stored NOx removed.
The inventors herein have recognized that a system and method are needed for vaporizing a reductant prior to supplying the reductant to an emission control device such as a NOx trap for optimal NOx reduction.
The foregoing problems and disadvantages are overcome by a system and method for removing NOx from an emission control device in accordance with the present invention.
The inventive method is implemented in an engine having an emission control device coupled adjacent and downstream of an oxidation catalyst. The method includes adding a reductant to the exhaust gases flowing into the oxidation catalyst. The method further includes partially oxidizing the reductant in the oxidation catalyst to transition a remaining portion of the reductant into a vapor phase. The method further includes oxidizing the remaining portion of the reductant in the emission control device to remove NOx from the device.
A system for removing NOx from an emission control device is also provided. The system includes an oxidation catalyst receiving exhaust gases from an engine. The oxidation catalyst is coupled adjacent and upstream of the emission control device. The system further includes a valve selectively supplying reductant to the exhaust gases flowing into the oxidation catalyst responsive to a first signal. The system further includes a controller operably connected to the valve. The controller generates the first signal to add the reductant to the exhaust gases flowing into the oxidation catalyst. The oxidizing catalyst partially oxidizes the reductant to obtain a remaining portion of the reductant in a vapor phase. The device oxidizes the remaining portion of the reductant to remove NOx from the device.
The inventive system and method provides a substantial advantage over known systems and methods. In particular, the inventive system and method vaporizes a reductant prior to supplying the reductant to an emission control device to increase NOx reduction by the device as compared with known systems.